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Maintainers of the Apache webserver are racing to patch a severe weakness that allows an attacker to use a single PC to completely crash a system and was first diagnosed 54 months ago.

Attack code dubbed “Apache Killer” that exploits the vulnerability in the way Apache handles HTTP-based range requests was published Friday on the Full-disclosure mailing list. By sending servers running versions 1.3 and 2 of Apache multiple GET requests containing overlapping byte ranges, an attacker can consume all memory on a target system.

“The behaviour when compressing the streams is devastating and can end up in rendering the underlying operating system unusable when the requests are sent parallely,” Kingcope, the researcher credited with writing and publishing the proof-of-concept attack code wrote Wednesday on Apache's Bugzilla discussion list. “Symptoms are swapping to disk and killing of processes including but solely httpd processes.”

The denial-of-service attack works by abusing the routine web clients use to download only certain parts, or byte ranges, of an HTTP document from an Apache server. By stacking an HTTP header with multiple ranges, an attacker can easily cause a system to malfunction. On Wednesday morning, Apache developers said they expect to release a patch in the next 96 hours.

The Apache advisory contains several workarounds that admins can deploy in the meantime.

The susceptibility of Apache's range handling to crippling DoS attacks was disclosed in January 2007 Michal Zalewski, a security researcher who has since taken a job with Google. He said at the time that both Apache and Microsoft's competing IIS webserver were vulnerable to crippling DoS attacks because of the programs' “bizarro implementation” of range header functionality based on the HTTP/1.1 standard.

“Combined with the functionality of window scaling (as per RFC 1323)), it is my impression that a lone, short request can be used to trick the server into firing gigabytes of bogus data into the void, regardless of the server file size, connection count, or keep-alive request number limits implemented by the administrator,” Zalewski wrote. “Whoops?”

In an email to The Register on Wednesday, Zalewski wrote: “Not sure why they haven't done something about it back then, probably just haven't noticed in absence of an exploit.”

The episode challenges the conventional wisdom repeated by many proponents of open-source software that flaws in freely available software get fixed faster than in proprietary code because everyday users are free to inspect the source code and report any vulnerabilities they find. Assuming that claim is true, the four-year weakness in Apache's range-handling feature would appear to be an obvious exception.

About 235 million websites use Apache, making it the most widely used webserver with about 66 percent of the entire internet, according to figures released last month by Netcraft. IIS ranked second with more than 60 million sites, or about 17 percent.

In a statement issued several hours after this article was published, Microsoft spokesman Jerry Bryant said: "IIS 6.0 and later versions are not susceptible to this type of denial-of-service due to built in restrictions." ®

Update

Trustwave's SpiderLabs has provided a detailed technical analysis here along with instructions for mitigating attacks using the open-source ModSecurity firewall.